Grease and line 3 large baking trays. Whisk the butter and caster sugar in a bowl until pale and creamy. Whisk in the egg yolk and ½ tsp almond extract.
Sieve in the flour and baking powder, add the ground almonds and mix together until the mixture resembles a crumble. Using your hands, bring the mixture together to form a firm dough. On a lightly floured surface knead the dough until smooth and pliable.
Roll out the dough to a thickness of ½ cm. Using a 7cm wide butterfly shaped cutter stamp out cookies. Arrange on the baking trays, prick the cookies lightly with a fork and chill for 30 minutes.
Ten minutes before baking, preheat the oven to 180°C (160°C fan, 350°F/Gas 4). Bake the cookies for 10-12 minutes until firm and lightly golden. Leave to cool for 10 minutes before transferring to a wire rack to cool completely.
Sift the icing sugar into a bowl, adding the remaining almond extract and the egg whites. Mix until combined.
Carefully combine the wet and dry ingredients together to make a soft, spreadable icing – If it is too stiff add a few drops of water, or if too dry, add some sifted icing sugar, 1 tbsp at a time. Beat until smooth and glossy.
Put about 2 tbsp of the icing in a small bowl and add a little red food colouring to make a pale pink shade.
Spoon a further 2 tbsp of the white icing into a small piping bag fitted with a No: 2 size plain piping nozzle. Pipe a line approx. 1mm from the edge of all of the cookies, marking out the wings and body.
Working on one butterfly at a time, spoon ½ tsp of the white icing into the centre of each butterfly wing and gently spread it to fill the centre. Flood the centre of the biscuit with a little pink icing to make the body.
To finish, decorate the wings with shimmer pearls.